This invention relates to the manufacture of hollow ingots of fused quartz of the type which serve as the preform for the manufacture of high quality fused quartz or fused silica tubing.
In an older process, the manufacture of fused quartz tubing is accomplished without the production first of a preformed ingot. In this process, quartz material capable of being fused is filled into a vertical, high temperature refractory metal furnace and is gradually heated above the melting point of Cristobalite (1728.degree. C.), above which only amorphous SiO.sub.2 exists. The molten and soft material is then extruded at the bottom of the furnace while being shaped into a tube or rod depending on the type of orifice or die employed in the apparatus.
One major disadvantage of this process is that there is a severe limitation on the range of sizes of the tubes or rods which can be produced by this method. It is generally known that the maximum diameter of the tubing which can be produced is limited to a few inches, while many applications require tubing of significantly larger diameters.
Another limitation of this older process is the relatively low quality of the fused material, regardless of the grade of the fusing quartz material which is fed into the furnace. For example, it is known to those familiar with the art that such directly produced material is characterized by a comparatively high content of open and closed bubbles. Further, die marks often appear on the outside of tubes or rods and metal contamination frequently occurs due to the mutual exposure of the furnace lining with the molten quartz whenever the flow of inert gases is not closely enough controlled. However, the most severe limitation of this process is the lack of versatility of the apparatus with respect to the manufacture of different types of tubing and changes of dimensions during a single furnace run. Once the furnace is started up it must be operated around the clock until, sometimes after several weeks of continuously running the same size tubing, the expensive furnace lining has deteriorated to a point at which even a lower grade tubing or rod cannot be produced, thus finally warranting to shut down the furnace.
In order to overcome the limitations of this older process all large diameter tubing and higher quality small and medium diameter tubing are produced by using the following two steps: (1) a suitable preform of the fused material is developed in the form of a cylindrical ingot or billet and, (2) the ingot is drawn into a tube which offers a choice of different diameters, wall thicknesses and other characteristics. This method also permits the manufacture of small as well as large quantities through multiples of short furnace runs.
In an older two-step process which has many disadvantages a hollow ingot is produced by employing first an expensive high temperature refractory metal mandrel in a vertical or horizontal position, which is covered with expensive fused quartz tubing and protected by a flow of inert gases. After heating the rotating arrangement, fine crystalline quartz powder is then applied to the outer surface of the fused quartz tubing and layers of newly fused quartz are formed while both the gas burner and the powder feeder move along the tubing until the ingot is completed.